Magnetic shielding

Shielding is necessary to create a magnetic-field-free environment for sensitive instruments such as high-resolution electron microscopes. It is also needed for measurements of very weak magnetic fields, such as those emanating from the brain. Two strategies to eliminate static and low-frequency fields are passive shielding and active shielding. Passive shielding involves surrounding the shielded volume by one or more boxes of high permeability material such as permalloy, which diverts the flux. Superconducting shields, which allow no change of flux in the enclosed volume, may be used for small spaces. Active shielding uses a sensor to detect one component of the field, which is then compensated by a current flowing in a pair of Helmholtz coils. Three orthogonal pairs can cancel the magnetic field vector completely.
At high frequency (≥ 100 kHz) a simpler approach is to surround the shielded volume by a continuous wire mesh known as a Faraday cage. Induced currents in the conducting circuits tend to cancel the changes to which they are due.